Tick size
The tick size is the smallest step a price can move on an exchange. Every price in the order book is a whole number of ticks.
On the NVIDIA perpetual on Binance futures the tick is 0.01, so prices go 217.64, 217.65, 217.66 and never anything in between. A spread of one tick is the tightest a market can be.
Learn more: the order book: how a price is made.